Substituent effect on photo- and electroluminescence properties of heteroleptic cyclometalated platinum(II) complexes based on a 2-(dibenzo[b,d]furan-4-yl)pyridine ligand

Accepted Manuscript Substituent effect on photo- and electroluminescence properties of heteroleptic cyclometalated platinum(II) complexes based on a 2-(dibenzo[b,d]furan-4-yl)pyridine ligand Tatsuya Shigehiro, Qiang Chen, Shigeyuki Yagi, Prof., Takeshi Maeda, Hiroyuki Nakazumi, Yoshiaki Sakurai PII:

S0143-7208(15)00362-9

DOI:

10.1016/j.dyepig.2015.09.015

Reference:

DYPI 4926

To appear in:

Dyes and Pigments

Received Date: 12 June 2015 Revised Date:

30 August 2015

Accepted Date: 10 September 2015

Please cite this article as: Shigehiro T, Chen Q, Yagi S, Maeda T, Nakazumi H, Sakurai Y, Substituent effect on photo- and electroluminescence properties of heteroleptic cyclometalated platinum(II) complexes based on a 2-(dibenzo[b,d]furan-4-yl)pyridine ligand, Dyes and Pigments (2015), doi: 10.1016/j.dyepig.2015.09.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

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Graphical abstract

ACCEPTED MANUSCRIPT

Substituent effect on photo- and electroluminescence properties of heteroleptic cyclometalated platinum(II) complexes based on a 2-

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(dibenzo[b,d]furan-4-yl)pyridine ligand

Tatsuya Shigehiroa, Qiang Chena, Shigeyuki Yagia,*, Takeshi Maedaa, Hiroyuki Nakazumia,

a

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Yoshiaki Sakuraib

Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture

b

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University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan Textile and Polymer Section, Technology Research Institute of Osaka Prefecture, 2-7-1

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Corresponding author:

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Ayumino, Izumi, Osaka 594-1157, Japan

Shigeyuki Yagi, Prof.

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TEL: +81-72-254-9324

FAX: +81-72-254-9910

E-mail: [email protected]

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ACCEPTED MANUSCRIPT Abstract A series of heteroleptic cyclometalated platinum(II) complexes based on a 4- (Pt-1) and 5substituted (Pt-2) cyclometalated ligand were prepared, and their photoluminescence (PL) and

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electroluminescence (EL) properties were investigated. In dichloromethane, the complexes bearing a fluoro (Pt-1a and Pt-2a) or a methyl (Pt-1b and Pt-2b) substituent exhibited green PL, whereas a trifluoromethyl substituent induced a red shift, affording yellow PL (Pt-1c and Pt-2c). For poly(methyl methacrylate) thin films doped with Pt-1a‒c, higher PL quantum

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yields were obtained in comparison with the solution samples. Polymer light-emitting diodes

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(PLEDs) containing the platinum(II) complexes were fabricated, and the devices doped with Pt-1b and Pt-2b showed relatively high external quantum efficiencies. PLEDs doped with varying concentrations of Pt-1b were also fabricated, where generation of excimer-based EL led to significant shifts of the Commision Internationale de L’Eclairage chromaticity

constant.

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coordinate from (0.40, 0.56) (green) to (0.51, 0.48) (orange) with the device efficiency almost

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Keywords: platinum complex; phosphorescence; photoluminescence; electroluminescence;

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organic light-emitting diode; excimer

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ACCEPTED MANUSCRIPT 1. Introduction In recent years, organic light-emitting diodes (OLEDs) have attracted enormous interest because of their strong potential for applications in next-generation flexible flat-panel displays [1‒3] and illumination devices [4‒6]. In particular, solution-processed polymer light-emitting

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diodes (PLEDs) [7‒9] have been receiving considerable attention due to availability of largearea devices, low-cost device fabrication, and efficient use of constituent materials. To obtain high performance PLEDs, phosphorescent organometallic complexes such as cyclometalated

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platinum(II) [10‒13] and iridium(III) [14‒17] complexes have often been employed as

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emitting dopants because the triplet excitons statistically generated via hole-electron recombination are combined with those from the excited singlet state via efficient intersystem crossing to achieve internal quantum efficiencies up to 100% [18‒20]. Among them, heteroleptic cyclometalated platinum(II) complexes have been eagerly studied along with heteroleptic bis- and homoleptic tris-cyclometalated iridium(III) complexes, and have often

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been employed as emitting dopants for PLEDs [21,22]. In general, the platinum(II) complexes adopt four-coordinated planar structures and often form excimers owing to strong intermolecular stacking interaction [10,23‒25]. The excimer-based luminescence affords a

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broad emission band at longer wavelength together with the original monomer emission. Thus,

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combining the monomer and excimer emission, single-doped white OLEDs (WOLEDs) have so far been reported [26,27]. Recently, we reported that an aromatic ancillary ligand 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate (bdbp) significantly facilitates the generation of excimer-based emission of

a

heteroleptic

complex

bearing

a

2-(dibenzo[b,d]furan-4-yl)pyridinate

(dbfp)

cyclometalated ligand, where various emission colors from green to reddish orange were obtained from the single-doped PLEDs by varying the doping level [28]. However, the excimer formation deteriorated device efficiencies due to facilitation of nonradiative decay via

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ACCEPTED MANUSCRIPT excimers as well as the decrease in carrier densities [29,30]. To seek for more efficient monomer and excimer emission from dbfp-based cyclometalated platinum(II) complexes, we focus on how the substituent on the dbfp cyclometalated ligand affects the emission quantum yield as well as the emission color. We here investigate luminescent properties of Pt-1 and Pt-

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2 (Fig. 1), where a fluorine, methyl, or trifluoromethyl is introduced at the 4- or 5-position of the cyclometalated ligand. Using an acetyleacetonate (acac) ancillary ligand in place of bdbp should allow us to estimate how the cyclometalated ligand affects the luminescent properties.

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We report the substituent effect on their photoluminescence (PL) behavior in solutions as well as polymer films. We also demonstrate the electroluminescence (EL) behavior of the PLEDs

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doped with Pt-1 and Pt-2. As Pt-1b exhibits excellent solubility in organic solutions and polymer media, the excimer emission behavior of PLEDs doped with Pt-1b is discussed.

2. Experimental Section

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Insert Fig. 1 here.

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2.1. General procedures

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All synthetic reagents except for potassium tetrachloroplatinate (K2PtCl4) were purchased from Wako Pure Chemical Industries Ltd., Tokyo Chemical Industry Co., Ltd., or SigmaAldrich Co., and used without further purification. K2PtCl4 was purchased from Furuya Metal Co., Ltd., and silica gel (spherically shaped, neutral) for flash column chromatography was purchased from Kanto Chemical Co., Inc. The preparation of the unsubstituted dbfp ligand has already been reported [13]. 1H (400 MHz) and 13C (100 MHz) NMR spectra were taken on a JEOL ECS-400 spectrometer, using TMS (0.00 ppm) as an internal standard. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra were measured on a

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ACCEPTED MANUSCRIPT Shimadzu-Kratos AXIMA-CFR PLUS TOF mass spectrometer using sinapinic acid as a matrix. Elemental analyses were carried out on a J-Science MICRO CORDER JM10 analyzer.

2.2. Preparation of materials

dbfp-based ligand

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2.2.1. 2-(Dibenzo[b,d]furan-4-yl)-4-fluoropyridine (4-F-dbfp-H): general procedure for the

A mixture of 2-chloro-4-fluoropyridine (0.580 g, 4.43 mmol) and Pd(PPh3)4 (0.172 g, 0.149 in

1,2-dimethoxyethane

(21

mL)

was

added

a

solution

of

4-

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mmol)

(dibenzo[b,d]furanyl)boronic acid (0.960 g, 4.51 mmol) in ethanol (21 mL), followed by

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addition of 2.0 M sodium carbonate aqueous solution (21 mL). Then, the mixture was refluxed for 1 day under nitrogen. After cooling, the solvent was removed on a rotary evaporator. The residue was dissolved in chloroform (50 mL), and then the solution was washed with water (50 mL × 2) and sat. brine (50 mL). The obtained organic solution was

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dried over anhydrous magnesium sulfate. The solvent was removed on a rotary evaporator, and the residue was purified by silica gel column chromatography using chloroform as eluent to obtain 2-(dibenzo[b,d]furan-4-yl)-4-fluoropyridine as a white powder (0.780 g, 2.96 mmol,

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67%); 1H NMR (400 MHz, CDCl3) δ 7.06 (m, 1H), 7.45‒7.51 (m, 2H), 7.60 (t, J = 7.6 Hz, 1H), 7.69 (dd, J = 2.4 and 9.6 Hz, 1H), 7.88‒7.93 (m, 1H), 7.96 (dd, J = 0.9 and 7.3 Hz, 1H),

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8.21 (m, 2H), 8.28 (dd, J = 0.9 and 7.3 Hz, 1H), 8.83 (dd, J = 6.2 and 9.6 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 110.21, 110.37, 111.94, 112.01, 112.19, 120.78, 122.00, 123.22, 125.38, 127.29, 127.55, 151.81, 151.89, 153.70, 156.11, 167.94, 170.54; MALDI-TOF mass m/z 263 ([M]+); Anal. Calcd for C17H10FNO: C, 77.56; H, 3.83; N, 5.32. Found: C, 77.59; H, 3.98; N, 5.23.

2.2.2. 2-(Dibenzo[b,d]furan-4-yl)-4-methylpyridine (4-CH3-dbfp-H)

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ACCEPTED MANUSCRIPT This compound was prepared according to the same procedure as 2-(dibenzo[b,d]furan-4yl)-4-fluoropyridine using 2-bromo-4-methylpyridine (yield, 94%); 1H NMR (400 MHz, CDCl3) δ 2.51 (s, 3H), 7.13 (d, J = 0.9 and 5.0 Hz, 1H), 7.38 (dt, J = 0.9 and 7.9 Hz, 1H), 7.46‒7.51 (m, 2H), 7.66 (d, J = 7.9 Hz, 1H), 8.00 (m, 2H), 8.19 (d, J = 0.9 Hz, 1H), 8.22 (dd, 13

C NMR (100 MHz, CDCl3) δ 18.40,

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J = 0.9 and 7.9 Hz, 1H), 8.64 (d, J = 5.0 Hz, 1H);

111.87, 120.73, 120.93, 122.98, 123.38, 123.78, 124.16, 124.41, 125.17, 127.11, 127.28, 132.07, 137.18, 150.32, 151.71, 153.70, 156.17; MALDI-TOF mass m/z 260 ([M + H]+);

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Anal. Calcd for C18H13NO: C, 83.37; H, 5.05; N, 5.40. Found: C, 83.04; H, 5.27; N, 5.34.

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2.2.3. 2-(Dibenzo[b,d]furan-4-yl)-4-(trifluoromethyl)pyridine (4-CF3-dbfp-H) This compound was prepared according to the same procedure as 2-(dibenzo[b,d]furan-4yl)-4-fluoropyridine using 2-bromo-4-(trifluoromethyl)pyridine (yield, 89%); 1H NMR (400 MHz, CDCl3) δ 7.40 (td, J = 0.9 and 7.9 Hz, 1H), 7.49‒7.54 (m, 3H), 7.68 (d, J = 7.9 Hz, 1H),

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8.01 (d, J = 7.9 Hz, 1H), 8.06 (dd, J = 0.9 and 7.6 Hz, 1H), 8.33 (dd, J = 0.9 and 7.6 Hz, 1H), 8.69 (d, J = 0.9 Hz, 1H), 8.95 (d, J = 5.0 Hz, 1H);

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C NMR (100 MHz, CDCl3) δ 112.02,

117.92, 117.95, 119.77, 119.80, 120.79, 122.24, 123.28, 123.44, 125.50, 127.32, 127.61,

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138.87, 139.21, 150.60, 153.73, 155.18, 156.10; MALDI-TOF mass m/z 313 ([M]+); Anal.

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Calcd for C18H10F3NO: C, 69.01; H, 3.22; N, 4.47. Found: C, 69.31; H, 3.50; N, 4.21.

2.2.4. 2-(Dibenzo[b,d]furan-4-yl)-5-fluoropyridine (5-F-dbfp-H) This compound was prepared according to the same procedure as 2-(dibenzo[b,d]furan-4yl)-4-fluoropyridine using 2-chloro-5-fluoropyridine (yield, 60%); 1H NMR (400 MHz, CDCl3) δ 7.38 (t, J = 7.6 Hz, 1H), 7.49 (m, 2H), 7.59 (m, 1H), 7.63 (d, J = 7.6 Hz, 1H), 7.99‒8.01 (m, 2H), 8.29 (dd, J = 0.9 and 7.6 Hz, 1H), 8.46 (dd, J = 4.6 and 8.6 Hz, 1H), 8.63 (d, J = 2.7 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 111.85, 120.82, 121.32, 123.15, 123.42,

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ACCEPTED MANUSCRIPT 124.05, 125.10, 127.21, 127.45, 137.96, 138.19, 150.04, 150.08, 153.49, 156.12, 157.51, 160.07; MALDI-TOF mass m/z 264 ([M + H]+); Anal. Calcd for C17H10FNO: C, 77.56; H, 3.83; N, 5.32. Found: C, 77.56; H, 3.99; N, 5.22.

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2.2.5. 2-(Dibenzo[b,d]furan-4-yl)-5-methylpyridine (5-CH3-dbfp-H)

This compound was prepared according to the same procedure as 2-(dibenzo[b,d]furan-4yl)-4-fluoropyridine using 2-bromo-5-methylpyridine (yield, 83%); 1H NMR (400 MHz,

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CDCl3) δ 2.44 (s, 3H), 7.37 (dt, J = 0.9 and 7.6 Hz, 1H), 7.48 (m, 2H), 7.66 (d, J = 7.6 Hz, 1H), 7.68 (dd, J = 1.9 and 7.8 Hz, 1H), 7.99 (m, 2H), 8.23 (dd, J = 0.9 and 7.6 Hz, 1H), 8.30

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(d, J = 7.8 Hz, 1H), 8.62 (d, J = 1.9 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 21.48, 111.93, 120.73, 121.15, 123.00, 123.33, 123.57, 124.15, 124.54, 124.88, 125.20, 127.29, 127.52, 147.73, 149.60, 153.73, 153.80, 156.18; MALDI-TOF mass m/z 260 ([M + H]+); Anal. Calcd

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for C18H13NO: C, 83.37; H, 5.05; N, 5.40. Found: C, 83.47; H, 5.43; N, 5.57.

2.2.6. 2-(Dibenzo[b,d]furan-4-yl)-5-(trifluoromethyl)pyridine (5-CF3-dbfp-H) This compound was prepared according to the same procedure as 2-(dibenzo[b,d]furan-4-

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yl)-4-fluoropyridine using 2-bromo-5-(trifluoromethyl)pyridine (yield, 83%); 1H NMR (400 MHz, CDCl3) δ 7.46‒7.52 (m, 2H), 7.63 (t, J = 7.8 Hz, 1H), 7.93 (m, 1H), 8.06 (m, 2H), 8.12

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(d, J = 7.8 Hz, 1H), 8.22 (m, 1H), 8.32 (dd, J = 0.9 and 7.8 Hz, 1H), 9.14 (m, 1H); 13C NMR (100 MHz, CDCl3) δ 120.34, 121.50, 122.56, 123.26, 124.41, 124.68, 125.58, 127.12, 131.88, 133.83, 133.86, 134.62, 137.64, 138.28, 141.98, 145.50, 145.54, 159.41; MALDI-TOF mass m/z 314 ([M + H]+); Anal. Calcd for C18H10F3NO: C, 69.01; H, 3.22; N, 4.47. Found: C, 69.31; H, 3.40; N, 4.43.

2.2.7. General procedure for mononuclear platinum(II) complexes ((X-dbfp)PtCl(X-dbfp-H),

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ACCEPTED MANUSCRIPT X = 4(5)-F, 4(5)-CH3, 4(5)-CF3) The mononuclear platinum(II) complexes (X-dbfp)PtCl(X-dbfp-H) were prepared as precursors for the target cyclometalated complexes, according to the reported procedure [13]. To a mixture of the corresponding dbfp-based ligand in 2-ethoxyethanol was added K2PtCl4,

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and then the mixture was heated at 80ºC for 1 day under nitrogen. After cooling, a plenty of water was added, and then the precipitate was collected by filtration under reduced pressure. The obtained precipitate was washed with ethanol and hexane to afford the mononuclear

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platinum(II) complexes (yields; 80‒99%). These compounds were poorly soluble, and so used

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in the next step without purification.

2.2.8. (2-(Dibenzo[b,d]furan-4-yl)-4-fluoropyridinato-N,C3’)platinum(II)(pentane-2,4dionate-O,O) (Pt-1a): general procedure for Pt-1‒3

A mixture of the corresponding mononuclear complex (4-F-dbfp)PtCl(4-F-dbfp-H) (0.700

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g, 0.926 mmol), acetylacetone (0.0760 g, 0.760 mmol), and sodium carbonate (0.250 g, 2.36 mmol) in 2-ethoxyethanol (35 mL) was stirred at 100ºC for 3 h under nitrogen atmosphere. The residue was dissolved in chloroform (50 mL), and then the solution was washed with

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water (50 mL × 2) and sat. brine (50 mL). The obtained organic solution was dried over anhydrous sodium sulfate. The solvent was removed on a rotary evaporator, and the residue

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was purified by silica gel column chromatography using chloroform/hexane (2/3, v/v) as eluent to obtain Pt-1a as a yellow powder (0.148 g, 0.266 mmol, 58%); 1H NMR (400 MHz, CDCl3) δ 2.02 (s, 3H), 2.05 (s, 3H), 5.50 (s, 1H), 6.95 (td, J = 2.7 and 6.8 Hz, 1H), 7.43‒7.49 (m, 2H), 7.80‒7.88 (m, 3H), 8.03 (d, J = 8.1 Hz, 1H), 8.15‒8.17 (m, 1H), 9.12 (t, J = 6.8 Hz, 1H);

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C NMR (100 MHz, CDCl3) δ 27.28, 28.31, 102.79, 108.59, 108.79, 109.15, 109.37,

120.99, 122.43, 122.57, 124.94, 126.22, 127.22, 133.60, 134.88, 135.73, 137.09, 141.09, 149.81, 149.90, 184.47, 186.03; MALDI-TOF mass m/z 556 ([M]+). Anal. Calcd for

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ACCEPTED MANUSCRIPT C22H16FNO3Pt: C, 47.49; H, 2.90; N, 2.52. Found: C, 47.79.; H, 3.09; N, 2.72.

2.2.9. (2-(Dibenzo[b,d]furan-4-yl)-4-methylpyridinato-N,C3’)platinum(II)(pentane-2,4dionate-O,O) (Pt-1b)

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This complex was prepared from (4-CH3-dbfp)PtCl(4-CH3-dbfp-H) according to the same procedure as Pt-1a (yield, 68%); 1H NMR (400 MHz, CDCl3) δ 2.01 (s, 3H), 2.04 (s, 3H), 2.56 (s, 3H), 5.49 (s, 1H), 6.98 (m, 1H), 7.33 (dt, J = 0.9 and 7.3 Hz, 1H), 7.42 (dt, J = 0.9 and

(d, J = 6.4 Hz, 1H);

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7.3 Hz, 1H), 7.62 (m, 2H), 7.79 (d, J = 7.8 Hz, 1H), 7.92 (d, 7.3 Hz, 1H), 8.38 (s, 1H), 8.86 C NMR (100 MHz, CDCl3) δ 21.90, 27.26, 28.32, 102.66, 111.40,

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120.04, 120.77, 121.18, 121.95, 122.92, 123.09, 124.85, 124.90, 126.03, 129.35, 139.37, 146.59, 150.64, 152.60, 155.38, 165.77, 184.22, 185.80; MALDI-TOF mass m/z 553 ([M + H]+); Anal. Calcd for C23H19NO3Pt: C, 50.00; H, 3.47; N, 2.54. Found: C, 50.21; H, 3.49; N,

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2.55.

2.2.10. (2-(Dibenzo[b,d]furan-4-yl)-4-(trifluoromethyl)pyridinato-N,C3’)platinum(II)(pentane2,4-dionate-O,O) (Pt-1c)

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This complex was prepared from (4-CF3-dbfp)PtCl(4-CF3-dbfp-H) according to the same

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procedure as Pt-1a (yield, 62%); 1H NMR (400 MHz, CDCl3) δ 2.03 (s, 3H), 2.05 (s, 3H), 5.52 (s, 1H), 7.28 (dt, J = 1.8 and 6.2 Hz, 1H), 7.35 (dt, J = 0.9 and 7.8 Hz, 1H), 7.45 (dt, J = 0.9 and 7.8 Hz, 1H), 7.59 (d, J = 7.7 Hz, 1H), 7.61 (d, J = 8.3 Hz, 1H), 7.85 (d, J = 8.3 Hz, 1H), 7.93 (dd, J = 0.9 and 7.8 Hz, 1H), 8.67 (d, J = 1.8 Hz, 1H), 9.27 (d, J = 6.2 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 27.16, 28.29, 102.75, 111.62, 116.69, 118.07, 118.11, 120.06, 121.29, 122.10, 123.16, 124.48, 124.91, 126.33, 128.00, 140.20, 140.32, 147.97, 153.01, 155.36, 167.82, 184.33, 186.11; MALDI-TOF mass m/z 607 ([M + H]+). Anal. Calcd for C23H16F3NO3Pt: C, 45.55; H, 2.66; N, 2.31. Found: C, 45.52; H, 2.79; N, 2.23.

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ACCEPTED MANUSCRIPT

2.2.11. (2-(Dibenzo[b,d]furan-4-yl)-5-fluoropyridinate-N,C3’)platinum(II)(pentane-2,4dionate-O,O) (Pt-2a) This complex was prepared from (5-F-dbfp)PtCl(5-F-dbfp-H) according to the same

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procedure as Pt-1a (yield, 45%); 1H NMR (400 MHz, CDCl3) δ 2.05 (s, 3H), 2.06 (s, 3H), 5.51 (s, 1H), 7.34 (dt, J = 0.9 and 7.8 Hz, 1H), 7.43 (dt, J = 0.9 and 7.8 Hz, 1H), 7.57‒7.61 (m, 2H), 7.72‒7.74 (m, 1H), 7.81 (d, J = 7.8 Hz, 1H), 7.92 (dd, J = 0.9 and 7.8 Hz, 1H), 8.58

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(dd, J = 5.5 and 9.2 Hz, 1H), 9.01 (t, J = 2.3 Hz, 1H); MALDI-TOF mass m/z 556 ([M]+). Anal. Calcd for C22H16FNO3Pt: C, 47.49; H 2.90; N, 2.52. Found: C, 47.32; H, 3.08; N, 2.37.

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The 13C NMR spectrum was not obtained due to the low solubility.

2.2.12. 5-Methyl-2-(dibenzo[b,d]furan-4-yl)pyridinate-N,C3’)platinum(II)(pentane-2,4dionate-O,O) (Pt-2b)

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This complex was prepared from (5-CH3-dbfp)PtCl(5-CH3-dbfp-H) according to the same procedure as Pt-1a (yield, 60%); 1H NMR (400 MHz, CDCl3) δ 2.04 (s, 3H), 2.05 (s, 3H), 2.45 (s, 3H), 5.50 (s, 1H), 7.32 (dt, J = 0.9 and 7.8 Hz, 1H), 7.41 (dt, J = 0.9 and 7.8 Hz, 1H),

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7.58‒7.60 (m, 1H), 7.61 (d, J = 7.8 Hz, 1H), 7.74‒7.76 (m, 1H), 7.78 (d, J = 7.8 Hz, 1H), 7.92 (d, J = 0.9 and 7.8 Hz, 1H), 8.46 (d, J = 8.3 Hz, 1H), 8.86 (d, J = 1.6 Hz, 1H); 13C NMR (100

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MHz, CDCl3) δ 18.49, 27.26, 28.41, 102.69, 111.37, 120.08, 120.44, 121.24, 121.87, 122.89, 124.76, 124.86, 126.04, 129.44, 131.16, 138.58, 139.34, 147.24, 152.38, 155.42, 163.71, 184.31, 185.90; MALDI-TOF mass m/z 553 ([M + H]+). Anal. Calcd for C23H19NO3Pt: C, 50.00; H, 3.47; N, 2.54. Found: C, 50.30; H, 3.57; N, 2.53.

2.2.13. (2-(Dibenzo[b,d]furan-4-yl)-5-(trifluoromethyl)pyridinate-N,C3’)platinum(II)(pentane2,4-dionate-O,O) (Pt-2c)

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ACCEPTED MANUSCRIPT This complex was prepared from (5-CF3-dbfp)PtCl(5-CF3-dbfp-H) according to the same procedure as Pt-1a, affording a yellow powder (yield, 65%); 1H NMR (400 MHz, CDCl3) δ 2.05 (s, 3H), 2.08 (s, 3H), 5.52 (s, 1H), 7.44‒7.50 (m, 2H), 7.81 (d, J = 8.2 Hz, 1H), 7.85‒7.88 (m, 1H), 8.03 (d, J = 8.2 Hz, 1H), 8.11‒8.18 (m, 3H), 9.38 (s, 1H);

13

C NMR (100 MHz,

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CDCl3) δ 27.23, 28.32, 102.85, 120.70, 120.99, 122.40, 123.17, 123.34, 123.68, 124.07, 125.01, 126.28, 127.24, 133.59, 135.37, 135.66, 136.81, 137.04, 143.71, 144.69, 170.58, 184.43, 186.40; MALDI-TOF mass m/z 607 ([M + H]+). Anal. Calcd for C23H16F3NO3Pt: C,

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45.55; H, 2.66; N, 2.31. Found: C, 45.21; H, 2.69; N, 2.05.

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2.2.14. (2-(Dibenzo[b,d]furan-4-yl)pyridinato-N,C3’)platinum(II)(pentane-2,4-dionate-O,O) (Pt-3)

This complex was prepared from (5-dbfp)PtCl(5-dbfp-H) [13] according to the same procedure as Pt-1a, affording a yellow powder (yield, 83%); 1H NMR (400 MHz, CDCl3) δ

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2.02 (s, 3H), 2.05 (s, 3H), 5.54 (s, 1H), 7.15 (ddd, J = 1.4, 5.9 and 7.8 Hz, 1H), 7.33 (td, J = 1.3 and 7.6 Hz, 1H), 7.43 (td, J = 1.3 and 7.6 Hz, 1H), 7.58 (d, J = 8.3 Hz, 1H), 7.62 (d, J = 7.6 Hz, 1H), 7.80 (d, J = 8.3 Hz, 1H), 7.91‒7.95 (m, 2H), 8.57 (td, J = 0.9 and 7.8 Hz, 1H),

EP

9.05 (dt, J = 0.9 and 5.9 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 27.17, 28.26, 29.71, 102.63,

AC C

111.31, 120.01, 120.88, 120.97, 121.18, 122.30, 122.88, 124.70, 124.80, 126.03, 138.60, 147.26, 155.33, 166.44, 184.26, 185.86; MALDI-TOF mass m/z 538 ([M]+). Anal. Calcd for C22H17NO3Pt: C, 49.07; H, 3.18; N, 2.60. Found: C, 48.83; H, 2.51; N, 3.33.

2.3. Measurements UV-vis absorption spectra were recorded on a Shimadzu UV-3600 spectrometer. PL spectra were recorded on a Horiba SPEX Fluorolog-3 spectrofluorometer. PL quantum yields were measured on a Hamamatsu Photonics C9920-12 absolute PL quantum yield measurement

11

ACCEPTED MANUSCRIPT system. PL lifetimes were obtained on a Horiba Jobin Yvon FluoroCube spectroanalyzer using a 390 nm nanosecond-order LED light source. The optical and photophysical measurements were carried out under nitrogen atmosphere just after sample preparation. The solution samples were subjected to nitrogen bubbling for at least five minutes followed by complete

RI PT

sealing. The poly(methyl methacrylate) (PMMA; Mw, not reported) films were prepared using commercially available PMMA from Wako Pure Chemical Industries, Ltd. just as obtained.

For

fabrication

of

PLEDs,

SC

2.4. Fabrication of PLEDs

poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

M AN U

(PEDOT:PSS; Clevios P VP CH8000, Heraeus GmbH), 2-(4-biphenylyl)-5-(4-tertbutylphenyl)-1,3,4-oxadiazole (PBD; Tokyo Chemical Industry Co., Ltd.), cesium fluoride (Wako Pure Chemical Industries, Ltd.), and aluminum wires (Nilaco Corporation) were purchased and used as received. Poly(9-vinylcarbazole) (PVCz; Mw = 25,000–50,000) was

TE D

purchased from Sigma-Aldrich Co., and used after purification by reprecipitation from THF to methanol. All the cyclometalated platinum(II) complexes were thoroughly purified by recrystallization from ethyl acetate or reprecipitation from chloroform-hexane before device

EP

fabrication. Indium-tin oxide (ITO) glass substrates with a sheet resistance (10 Ω/square) were purchased from Sanyo Vacuum Industries Co., Ltd. The PLED fabrication was carried out in

AC C

the same way as the previous report [28]. The device fabrication was carried out in a glove box filled with dry argon, except for the preparation of PEDOT:PSS layer. The PLED performance was operated at room temperature, using a Hamamatsu Photonics C-9920-11 organic EL device evaluating system.

3. Results and Discussion 3.1. Synthesis

12

ACCEPTED MANUSCRIPT The platinum(II) complexes Pt-1‒3 were synthesized according to the reported procedure (Scheme 1) [13]. Each dbfp-based ligand X-dbfp-H (X; 4(5)-F, 4(5)-CH3, 4(5)-CF3) was prepared

by

Suzuki-Miyaura

cross-coupling

reaction

of

commercially

available

dibenzo[b,d]furan-4-ylboronic acid and the corresponding 2-halopyridine. The obtained

RI PT

ligands were subsequently reacted with K2PtCl4 to afford the mononuclear platinum(II) complexes (X-dbfp)PtCl(X-dbfp-H) as precursors, which were reacted with acetylacetone in the presence of sodium carbonate to afford Pt-1‒3. The structures of these complexes were

SC

identified by 1H and 13C NMR, MALDI-TOF mass as well as elemental analysis.

M AN U

Insert Scheme 1 here.

3.2. UV-vis absorption and PL properties

In Fig. 2 are shown UV-vis absorption and PL spectra of Pt-1 and Pt-2 in dichloromethane

TE D

at rt. The PL data for Pt-1‒3 in dichloromethane are also summarized in Table 1. In the UVvis absorption spectra of Pt-1a and Pt-1b, the structured bands with relatively large molar absorptivities (ε; 11000‒42700 M-1cm-1) at < ca. 350 nm are assigned to π-π* transitions on

EP

the dbfp-based ligands. The broad absorption bands with smaller ε (5600‒6900 M-1cm-1) in

AC C

the region of > ca. 390 nm are also observed, assignable to singlet metal-to-ligand charge transfer (MLCT) transitions. In the present case, any remarkable triplet transitions are not observed. These spectral features are consistent with that of the unsubstituted complex Pt-3, whereas the lower-energy band of Pt-1c is somewhat red-shifted. In the case of Pt-2, a similar tendency to Pt-1 is observed: the 5-fluorine- and 5-methyl-substituted complexes (Pt-2a and Pt-2b, respectively) show similar spectral feature to Pt-3, whereas 5-trifluoromethyl substituted complex Pt-2c shows red-shifted absorption as seen in Pt-1c.

13

ACCEPTED MANUSCRIPT Insert Fig. 2 and Table 1 here.

In the PL spectra (Fig. 2), the emission maxima (λPLs) of Pt-1a and Pt-1b are observed at 518 and 509 nm, respectively, exhibiting green PL as is observed for Pt-3 (λPL; 515 nm). In

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the case of Pt-1c, however, the introduction of a trifluoromethyl group gives rise to a ca. 30 nm red shift of λPL, as is expected from the red shift in the UV-vis absorption spectra: Pt-1c exhibits yellow PL. In the case of the 5-substituted complexes, Pt-2c exhibits red-shifted PL

SC

by 19‒24 nm in comparison with Pt-2a and Pt-2b, as is similar to Pt-1c. Thus, the introduction of a trifluoromethyl substituent to the pyridine moiety of bdfp gives larger

M AN U

electronic impact on the optical and PL properties than methyl and fluorine substituents. The PL quantum yields (ΦPLs) of Pt-1a‒c are not so improved (ΦPL; 0.24‒0.34) in comparison with the unsubstituted complex Pt-3 (ΦPL; 0.24), whereas those of Pt-2a‒c are improved at some extent (ΦPLs; 0.41‒0.47).

TE D

The UV-vis absorption and PL properties of Pt-1‒3 in polymer thin films were also investigated, where PMMA was employed as a matrix polymer because the absence of

EP

absorption bands from near-UV to visible regions allows us to estimate the intrinsic PL properties of the platinum(II) complexes [28]. In PMMA films, UV-vis absorption and PL

AC C

properties of Pt-1a‒c and Pt-2a‒c showed almost the same profiles as those in dichloromethane solutions (Table 1 and Fig. 3). This result indicates that any interactions causing the aggregation and/or excimer formation are negligible at the present doping level (0.1 wt%). However, ΦPLs of Pt-1a‒c were significantly improved (ΦPL; 0.70‒0.75) in comparison with those in dichloromethane solutions, although Pt-2a‒c showed little changes except for Pt-2c (ΦPL; 0.72). Therefore, taking the ΦPL of Pt-3 (ΦPL; 0.51) into consideration, the introduction at the 4-position is one of the convenient ways to enhance emission efficiencies of dbfp-based cyclometalated platinum(II) complexes in a polymer medium.

14

ACCEPTED MANUSCRIPT

Insert Fig. 3 here.

3.3. TD-DFT study

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To elucidate the characteristic substituent effect of a trifluoromethyl group on the PL properties, time-dependent density functional theory (TD-DFT) calculations were performed at B3LYP/LANL2DZ for the platinum atom and B3LYP/6-31G* for the other atoms

SC

(implemented in Gaussian 09 package [31]), and the electronic structures of the lowest triplet excited states (T1) were obtained for Pt-1c, Pt-2c, and Pt-3. In Table 2 is listed the assignment

M AN U

of the calculated S0→T1 transition for these complexes, and in Fig. 4 are shown their frontier orbitals involved in the main compositions of the S0→T1 transition. As shown in Table 2, the S0→T1 transition of each complex is exclusively composed of the HOMO→LUMO transition (> 87%), where the characters of the intraligand charge transfer (ILCT) at the dbfp ligand, the

TE D

MLCT from the platinum atom to dbfp, and the ligand-to-ligand charge transfer (LLCT) from the acac to the dbfp are involved (Fig. 4). The other composition (HOMO-1→LUMO) makes a small contribution to the S0→T1 transition. The HOMO of Pt-1c is stabilized by 0.21 eV in

EP

comparison with that of Pt-3 (‒5.39 eV), whereas Pt-2c has almost the same HOMO level (‒5.35 eV) as Pt-3. On the other hand, the LUMO levels of Pt-1c and Pt-2c are stabilized

AC C

more than the corresponding HOMO levels; stabilization by 0.52 and 0.11 eV is estimated for Pt-1c and Pt-2c, respectively, in comparison with Pt-3. Obviously, the decrease in the S0→T1 transition energy in Pt-1c and Pt-2c should be attributed to the stabilization of the LUMO [32,33].

Insert Table 2 and Fig. 4 here.

3.4. EL properties of solution-processed PLEDs 15

ACCEPTED MANUSCRIPT Using Pt-1a‒c and Pt-2a‒c as an emitting dopant, PVCz-based PLEDs were fabricated. The device structure is as follows; ITO (anode, 150 nm)/PEDOT:PSS (40 nm)/emitting layer (EML, 100 nm)/CsF (1.0 nm)/Al (cathode, 250 nm), where EML consists of PVCz doped with the platinum(II) complex and PBD (an electron transporting material). The weight ratio

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of PVCz:PBD:Pt-1a‒c (or Pt-2a‒c) was adjusted to 1.0:0.30:0.10 (wt/wt/wt). The obtained EL spectra at the maximum luminance (Lmax) of PLEDs containing Pt-1a‒c and Pt-2a‒c are shown in Figs. 5a and 5b, respectively. The device performance is also listed in Table 3. The

SC

EL spectra of PLEDs were comparable to the PL spectra of the emitting dopants. The PLED using Pt-1a showed the emission maxima (λELs) at 524 and 563 nm, affording green EL with

M AN U

the Commision Internationale de L’Eclairage (CIE) chromaticity coordinate of (0.40, 0.58) (@Lmax). The PLED using Pt-1b showed a similar spectral profile to the Pt-1a-doped device, emitting green (CIE; (0.40, 0.56)). For the devices doped with Pt-2a and Pt-2b, similar green EL was observed, affording the CIE chromaticity coordinates of (0.41, 0.57) and (0.42, 0.55),

TE D

respectively (@Lmax). From the viewpoint of the device efficiencies, the PLED doped with the methyl-substituted complex Pt-1b (or Pt-2b) afforded relatively high performance in comparison with OLEDs containing heteroleptic cyclometalated platinum(II) complexes as a

EP

phosphorescent dopant [21,22,34‒37]; the maximum external quantum efficiency (ηext) of

AC C

4.56% (4.74%), the maximum current efficiency (ηj) of 14.0 cd A-1 (14.2 cd A-1), and the maximum power efficiency (ηp) of 4.43 lm W-1 (5.00 lm W-1). As expected from the PL spectra, Pt-1c- and Pt-2c-doped devices exhibited the red-shifted EL with the CIE chromaticity coordinates of (0.47, 0.53) and (0.48, 0.52), respectively (@Lmax), emitting yellow. However, the device performance of these PLEDs was deteriorated down to the ηexts of 0.87 and 1.78%, respectively. Especially, the Pt-1c-doped device showed quite low performance. This should be attributed to the highly stabilized HOMO of Pt-1c, as expected

16

ACCEPTED MANUSCRIPT from the TD-DFT study discussed above, and therefore the hole injection to the emitting dopant should get less efficient.

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Insert Fig. 5 and Table 3 here.

3.5. Excimer-based EL behavior

Among the developed platinum(II) complexes, Pt-1b exhibits enough solubility in organic

SC

solvents (e.g. up to 12 mM in toluene) to allow us to fabricate PLEDs at the high doping levels. Thus, we next investigated the excimer-based EL behavior of the PLEDs doped with

M AN U

Pt-1b. The device structure is as follows; ITO (anode, 150 nm)/PEDOT:PSS (40 nm)/emitting layer (EML, 100 nm)/CsF (1.0 nm)/Al (cathode, 250 nm), where EML consists of PVCz, PBD, and Pt-1b. The ratio of PVCz:PBD:Pt-1b was adjusted to 1.0:0.30:x (wt/wt/wt), where x varied from 0.10 (7.1 wt%) to 0.40 (24 wt%) via 0.20 (13 wt%) and 0.30 (19 wt%). In Fig. 6

TE D

are shown the EL spectra (@Lmax) upon varying the doping level of Pt-1b from 7.1 to 24 wt%, normalized at λPL of the monomer-based emission (@515 nm). The device performance is also summarized in Table 4. The increase in the doping level of Pt-1b gave rise to significant

EP

enhancement of the excimer-based EL, where the CIE chromaticity coordinates changed from (0.40, 0.56) (7.1 wt% doped) to (0.51, 0.48) (24 wt% doped). This means that emission color

AC C

tuning is available from green to orange, using only Pt-1b as an emitting dopant. It is worthy to note that the device efficiencies such as ηexts (Table 4) rarely declined upon varying concentrations, although the excimer formation generally deteriorates the device performance due to facilitating the non-radiative decay [29‒31]. With this respect, we investigated the excimer-based PL behavior of Pt-1b in PBD-PVCz blend films that consisted of the identical compositions with the emitting layers of the fabricated PLEDs. As shown in Fig. 7, the PL spectral changes were relatively modest in comparison with the EL spectral changes due to the

17

ACCEPTED MANUSCRIPT difference in the mechanism of triplet excimer formation [28]. However, the double exponential decay of the PL lifetime (detected at 600 nm) was accompanied with the increase in the shorter component as the doping level increased (Table 5), indicating that the spectral changes are surely caused by the generation of the excimer-based PL [28]. As seen in Table 5,

RI PT

it is interesting that the ΦPL was almost constant, regardless of the doping level (ΦPL; 0.63 and 0.57 at 7.1 and 24 wt%, respectively). Therefore, this is the reason why the Pt-1b-doped

SC

PLEDs give rise to various colors without any decreases in device efficiencies.

M AN U

Insert Fig. 6 and Table 4 here.

Insert Fig. 7 and Table 5 here.

4. Conclusions

TE D

We have developed a series of dbfp-based heteroleptic cyclometalated platinum(II) complexes, and investigated their luminescent properties in CH2Cl2 solutions and PMMA films. The complexes whose cyclometalated ligand was attached with a fluoro or methyl

EP

substituent at the 4- or 5-position (Pt-1a,b and Pt-2a,b) showed green PL, as observed in the

AC C

unsubstituted complex Pt-3. On the other hand, the introduction of a trifluoromethyl substituent afforded red-shifted PL in the yellow region, as seen in Pt-1c and Pt-2c. The TDDFT calculations indicated that the lower-energy shift of PL is attributed to the stabilization of the LUMOs of the complexes. In PMMA films, the 4-substituted complexes (Pt-1a‒c) showed improved ΦPLs (ΦPL; 0.70‒0.75), although those in CH2Cl2 were relatively low. Thus, it was found that the substitution at the 4-position is effective to obtain the dbfp-based complex with a large ΦPL in a polymer matrix. Using the developed complexes as emitting dopants, PLEDs were fabricated. Among them, the devices doped with the methyl-substituted

18

ACCEPTED MANUSCRIPT complexes showed relatively high device efficiencies (ηext; 4.56 and 4.74% for Pt-1b and Pt2b, respectively). Excimer-based EL behavior was also investigated for the Pt-1b-doped PLEDs, where varying the doping level of Pt-1b gave rise to emission color changes from green (monomer) to orange (excimer, exclusively) without little deterioration of the device

RI PT

efficiencies.

Acknowledgements

SC

This work was partially supported by JSPS Grant-in-Aid for Scientific Research (B) (no.

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24350101).

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Table 1

CH2Cl2 solution Complex

PMMA film

λabs / nm (εabs / Lmol-1cm-1)

λPL / nm

ΦPL

τPL / µs (χ2)

Pt-1a

296 (42700), 352 (11000), 404 (5600)

518, 554

0.30

1.14 (1.01)

Pt-1b

286 (42700), 336 (16200), 394 (6900)

509, 544

0.24

1.08 (1.04)

Pt-1c

293 (47900), 353 (17400), 428 (5100)

545, 583

0.34

0.609 (1.06)

Pt-2a

286 (38000), 344 (13500), 408 (3500)

520, 556

0.47

Pt-2b

283 (39800), 340 (14500), 396 (4900)

515, 552

Pt-2c

296 (35500), 369 (9800), 430 (3400)

Pt-3

285 (44700), 368 (6200), 400 (4700)

λPL / nm

ΦPL

2.39

516, 554

0.73

2.44

509, 544

0.75

2.28

545, 583

0.70

1.42 (1.08)

2.38

520, 556

0.48

0.45

1.73 (1.05)

2.41

515, 552

0.40

544, 584

0.41

1.28 (1.08)

2.28

544, 584

0.72

515, 550

0.24

0.744 (1.04)

2.41

515, 550

0.51

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SC

T1a / eV

EP

Triplet energy estimated from the lowest-energy peak of the PL spectrum in CH2Cl2.

AC C

a

RI PT

UV-vis Absorption and PL data of Pt-1‒3 in CH2Cl2 (10 µM) and PMMA films (0.1 wt% doped) at rt.

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Table 2

RI PT

S0→T1 transition energies, their electronic transition assignments, the character of HOMO→LUMO transition and HOMO/LUMO levels of the calculated low-lying triplet excited state based on T1

Complex

Pt-1c

S0→T1

Assignments

(eV) 1.85

HOMO → LUMO (88%) HOMO‒1 → LUMO (4%)

1.94

HOMO → LUMO (89%)

HOMO

LUMO

HOMO→LUMO transition

(eV)

(eV)

‒5.60

‒2.69

ILCT(π-π*), MLCT, LLCT

‒5.35

‒2.28

ILCT(π-π*), MLCT, LLCT

‒5.39

‒2.17

ILCT(π-π*), MLCT, LLCT

M AN U

Pt-2c

Characters of

SC

optimized geometry for Pt-1c, Pt-2c, and Pt-3.

HOMO‒1 → LUMO (4%) Pt-3

2.00

HOMO → LUMO (87%)

AC C

EP

TE D

HOMO‒1 → LUMO (5%)

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Table 3

ηextb /%

ηjb /cd A-1

ηpb /lm W-1

(@V)

(@V)

(@V)

(@V)

(0.40, 0.58)

5400 (12.5)

2.35 (7.0)

7.83 (7.0)

3.52 (7.0)

513, 552

(0.40, 0.56)

6200 (15.5)

4.56 (10.5)

14.0 (10.5)

4.43 (9.5)

Pt-1c

553, 577

(0.47, 0.53)

2600 (14.5)

0.87 (10.0)

2.78 (10.0)

0.86 (8.5)

Pt-2a

524, 563

(0.41, 0.57)

9100 (18.0)

3.89 (10.5)

12.7 (10.5)

3.98 (9.5)

Pt-2b

518, 556

(0.42, 0.55)

5300 (16.0)

4.74 (10.0)

14.2 (10.0)

5.00 (8.5)

Pt-2c

551, 583

(0.48, 0.52)

3800 (17.0)

1.78 (11.5)

5.41 (11.5)

1.46 (10.5)

Pt-3

518, 556

(0.40, 0.57)

6200 (14.5)

3.61 (9.0)

11.5 (9.0)

4.04 (8.5)

CIE (x, y)a

Pt-1a

524, 563

Pt-1b

Dopant

M AN U

λELa/nm

SC

Lmaxb /cd m-2

Emitting

RI PT

EL device performance of PLEDs containing Pt-1‒3 as an emitting dopant (7.1 wt% doped).

a

Obtained at the voltage where the maximum luminance (Lmax) was observed.

b

The maximum values of luminance (L), external quantum efficiency (ηext), current efficiency (ηj),

AC C

EP

TE D

and power efficiency (ηp). The voltages in the parentheses are the ones at which they were obtained.

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Table 4 Device performance of PLEDs containing varying concentrations of Pt-1b. ηextb /%

ηjb /cd A-1

ηpb /lm W-1

(@V)

(@V)

(@V)

(@V)

(0.46, 0.52)

5810 (16.5)

4.67 (11.0)

12.4 (11.0)

3.61 (11.0)

515, 560, 598

(0.49, 0.50)

5000 (15.5)

4.89 (10.5)

12.0 (11.0)

3.58 (9.0)

517, 569, 606

(0.51, 0.48)

3660 (17.0)

5.23 (11.5)

11.8 (11.5)

3.48 (10.0)

CIE (x, y)

13 wt%

515, 554, 594

19 wt% 24 wt%

Obtained at Lmax.

b

The maximum values.

AC C

EP

TE D

M AN U

a

a

SC

λELa/nm

RI PT

Lmaxb /cd m-2

Doping level

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Table 5 PL properties of Pt-1b in PBD-PVCz films at different dopant levels. τPL/µs [%] (χ2)b

513, 550

1.05 [6], 8.22 [94] (1.05)

13 wt%

514, 552

1.46 [11], 7.15 [89] (1.03)

19 wt%

515, 552

1.73 [21], 6.00 [79] (1.05)

24 wt%

515, 555

1.87 [23], 5.25 [77] (1.02)

Pt-1b in PBD-PVCz films was excited at 296 nm.

b

Pt-1b in PBD-PVCz films was excited at 390 nm.

AC C

EP

TE D

M AN U

a

ΦPLa

RI PT

λPL/nma

SC

Doping level 7.1 wt%

0.63

0.62

0.58

0.57

ACCEPTED MANUSCRIPT Scheme and Figure Captions

Scheme 1. Preparation of Pt-1‒3.

RI PT

Fig. 1. Structures of Pt-1‒3.

Fig. 2. UV-vis absorption and PL spectra of (a) Pt-1a‒c and (b) Pt-2a‒c in CH2Cl2 (10 µM) at

SC

rt.

M AN U

Fig. 3. UV-vis absorption (9.1 wt% doped) and PL spectra (0.1 wt% doped) of (a) Pt-1a‒c and (b) Pt-2a‒c in PMMA films at rt. For UV-vis measurements, the doping levels of Pt-1a and Pt-2a in PMMA films were no more than 9.1 wt% due to poor solubility.

TE D

Fig. 4. Frontier orbitals of Pt-1c, Pt-2c, and Pt-3 calculated at the B3LYP/LANL2DZ level of theory for the platinum atom and the B3LYP/6-31G* level of theory for the other atoms.

EP

Fig. 5. Normalized EL spectra of PLEDs at Lmax containing (a) Pt-1a‒c and (b) Pt-2a‒c as an

AC C

emitting dopant (7.1 wt% doped).

Fig. 6. Normalized EL spectra of PLEDs at Lmax containing varying concentrations of Pt-1b. (Dashed black line represents the EL spectrum at 7.1 wt%, extracted from Fig. 5a)

Fig. 7. Normalized PL spectra of PBD-PVCz films doped with varying concentration of Pt-1b.

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

Fig. 1.

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 2.

TE D

M AN U

SC

RI PT

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AC C

EP

Fig. 3.

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

Fig. 4.

ACCEPTED MANUSCRIPT

Pt -1a Pt -1b Pt -1c

600 700 Wavelength (nm)

800

Normalized EL intensity

(b)

600 700 Wavelength (nm)

TE D

500

M AN U

Pt -2a Pt -2b Pt -2c

EP

Fig. 5.

AC C

SC

500

RI PT

Normalized EL intensity

(a)

800

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

Fig. 6.

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

Fig. 7.

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

Scheme 1.

ACCEPTED MANUSCRIPT Highlight

►The 2-(dibenzo[b,d]furan-4-yl)pyridine-based platinum(II) complexes were prepared.

RI PT

►The introduction of a trifluoromethyl group gave rise to a red-shifted emission.

►The 4-substituted complexes showed improved PL quantum yields in the polymer films. ►PLEDs with the 4- and 5-methyl-substituted complexes showed good device performance.

AC C

EP

TE D

M AN U

SC

►Efficient excimer EL was obtained from the 4-methyl-substituted complex.